The invention relates to a method for obtaining line synchronization information items from a video signal, and also to an apparatus for carrying out the method.
The invention is based on a method for obtaining line synchronization information items from a video signal of the generic type of the independent claim 1. Even though digital system solutions will become increasingly predominant in future television technology, analogue source signals will still exist for many years in the future. Examples include the terrestrial reception of video signals, which is still widespread to date, and the analogue recording methods, e.g. according to the VHS standard in the case of video recorders. Such, analogue signal sources represent critical signal sources for digital systems, and their signal processing requires special measures.
To date, television receivers with digital signal processing (e.g. in the case of the 100 Hz technology) have been operating, as a rule, with clock systems which are synchronized with the respective input signal. Since the input signal is the analogue CVBS signal, either the horizontal sync pulse (line-locked clock) or, alternatively, the colour subcarriers or colour synchronizing pulses (burst) (colour subcarrier-locked clock) are frequently used as reference point for the synchronization. The sync separation in the video lines has usually been carried out to date by means of analogue methods using so-called sync separator stages and a PLL filter stage connected downstream. In television receivers with digital signal processing, a PLL filter stage which is a digital realization of the known analogue sync signal processing is usually used. The filter stage is then a digital PLL (Phase-Locked Loop). Examples of such digital PLL circuits are the circuits SAA 7111 from Philips, HMP 8112 from Harris and Digit 3000 from Intermetall. The principal problem with such digital PLL circuits is that the known instabilities in the picture occur when the input signal present is an analogue video signal picked off from an analogue video recorder which is currently operating in the search mode (fast forward or reverse run) . Many users of analogue video recorders are sufficiently acquainted with such instabilities. Specifically, disturbing horizontal stripes appear in the picture when the video recorder is operating in the search mode. These disturbing stripes originate from the fact that in the search mode, the video heads no longer run on a single slanted track but rather sweep across two or more slanted tracks, depending on the search speed. During the transition from one slanted track to the next, abrupt sudden phase changes arise with regard to the occurrence of the sync pulses of the video lines. These sudden phase changes are actually governed by the geometry in magnetic tape recording in accordance with the slanted track method. The sudden phase changes are therefore determined by the system and, in addition, virtually unavoidable.
Irregular occurrence of line sync pulses also arises, however, in the case of video signals generated by camcorders. In this case, the instabilities that occur are, as a rule, more severe than in the case of a normal video recorder, because the regulation of the head-drum speed is subject to greater fluctuations on account of the larger component tolerances.
EP-A 0 266 147 discloses a digital PLL circuit for a television receiver. In the case of this digital PLL circuit, in order to avoid the abovementioned problem in the search operating mode in video recorders, a switching unit is provided which drastically shortens the time constant of the phase-locked loop in the event of identification of a sudden phase change caused by the head changeover at the end of a slanted track, with the result that the region of instability in the picture is reduced in size. The disadvantage of this solution is that the reduction of the time constant of the phase-locked loop provided by this solution means that noise components in the video signal are able to be suppressed less well and disturbing lines still remain visible, even though to a lesser extent than when the time constant is larger.